1. Field of the Invention
The present invention relates to a transmission system comprising a transmitter coupled via a transmission channel to a receiver, in which the transmitter comprises a channel encoder for encoding source symbols into coded symbols, and in which the receiver comprises a source decoder for deriving reconstructed source symbols from the coded symbols received from the transmission channel, the transmission system comprising coding property setting means for setting at least one coding property of the channel encoder and the channel decoder.
2. Description of the Related Art
The present invention also relates to a transmitter and a receiver for use in such a transmission system. The invention further relates to a transmission method.
Such transmission systems can be used in applications where the quality of the transmission channel shows considerable variations. To enable a virtual error free transmission over such a transmission channel, in the transmitter the source symbols are encoded using a channel encoder according a code which has error correcting and/or error detecting capabilities. In the receiver the source symbols are reconstructed by a channel decoder. Useful codes can include convolutional codes and several types of block codes such as Reed-Solomon codes. Also a combination of a convolutional code with a block code is often used.
The ratio between the number of source symbols and the number of channel symbols of such a code is called the rate of the code. The error correction capabilities of such a code depend heavily on the rate of the code. In case of a transmission channel with a strongly varying transmission quality the rate of the used channel code should be chosen to obtain virtually error free transmission at the worst channel conditions. This leads to a loss of useful transmission capacity when the transmission quality is high.
To prevent this loss of transmission capacity, the transmission system can set the at least one coding property e.g. the rate of the channel encoder, in dependence on the transmission quality. It is further observed that it can be desirable to set the coding property of the channel encoder and decoder decoder in dependence of the type of source symbols to be transmitted. E.g. the transmission of data signals representing computer files, requires bit error rates below 10xe2x88x9210, and the transmission of digitized speech signal may require bit error rates only below 10xe2x88x924.
The problem to be solved by the present invention is how to inform the receiver of a change of the at least one property of the channel encoder, in order to be able to decode it.
To solve this problem the transmission system according to the present invention is characterized in that the transmitter comprises a further channel encoder for obtaining an encoded coding property from the at least one coding property, in that the transmitter is arranged for transmitting the encoded coding property to the receiver, in that the receiver comprises a further channel decoder for obtaining the at least one coding property from the encoded coding property, and in that the receiver is arranged for setting the at least one coding property of the channel decoder according to the at least one coding property.
By using a separate channel for transmitting the at least one coding property, it is obtained that the transmission of this coding property does not depend on the (main) channel coding scheme which might fail under rapidly changing channel conditions. Because in general the amount of information needed to communicate the at least one coding property is very small, a very strong error correcting coding scheme can be used in the further channel encoder.
An embodiment of the invention is characterized in that that the further channel encoder comprises a block encoder, and in that the further channel decoder comprises a block decoder.
The use of a combination of a block encoder and a block decoder enables an easy decoding, in particular when the amount of information to be transmitted is low.
A further embodiment of the present invention is characterized in that the encoded symbols are arranged in frames, and in that the transmitter is arranged for splitting the encoded coding property into at least two parts, and for transmitting said at least two parts in subsequent frames.
By splitting the encoded coding property in at least two parts which are transmitted in subsequent frames, it becomes possible to choose longer codewords in order to obtain a further increased error correcting capability.
A still further embodiment of the present invention is characterized in that the transmission system comprises transmission quality determining means for deriving a transmission quality measure from the channel decoder in the receiver, and means for transmitting via a further transmission channel the quality measure to the transmitter.
By using a return link from the receiver to the transmitter, it becomes easy to obtain a transmission quality at the transmitter. In a similar way, it is also possible to use transmission quality dependent channel encoding on a full duplex link.